In some fluid ejection devices, such as printheads, a drop ejecting element is formed on a front side of a substrate and fluid is routed to an ejection chamber of the drop ejecting element through an opening or slot in the substrate. Often, the substrate is a silicon wafer and the slot is formed in the wafer by chemical etching. Existing methods of forming the slot through the substrate include etching into the substrate from the backside of the substrate to the front side of the substrate. The backside of the substrate is defined as a side of the substrate opposite of which the drop ejecting element is formed. Unfortunately, etching into the substrate from the backside all the way to the front side may result in misalignment of the slot at the front side and/or varying width of the slot at the front side.
Accordingly, it is desired to control formation of the slot through the substrate.